Re: btusb "firmware request while host is not available" at resume

[Marcel Holtmann]

Hi Luis,

>>>> To confirm, reverting this fixes the problem I was seeing in 4.13.  I've
>>>> queued it up for the next 4.13-stable release as well.
>>> 
>>> Commit 81f95076281f ("firmware: add sanity check on shutdown/suspend") may
>>> seem kludgy but the reason for it was to cleanup the horrible forced and
>>> required UMH lock even when the UMH lock was *not* even needed, which was later
>>> removed via commit 06a45a93e7d34aa ("firmware: move umh try locks into the umh
>>> code").
>> 
>> So what does this mean now that it is reverted?
> 
> We discuss what we should do about upkeeping a warning in the future, as
> I think technically the warning was still valid and it could help avoid
> racy lookups with the filesystem which otherwise could have gone unnoticed.
> 
>>> Removing the old UMH lock even when the UMH lock was *not* needed was the right
>>> thing to do but commit 81f95076281f (("firmware: add sanity check on
>>> shutdown/suspend") was put in place as a safe guard as the lock was also
>>> placing an implicit sanity check on the API. It ensures the API with the cache
>>> was used as designed, otherwise you do run the risk of *not getting the
>>> firmware you may need* -- Marcel seems to acknowledge this possibility.
>>> 
>>> It may be possible for us to already have in place safeguards so that upon
>>> resume we are ensuring the path to the firmware *is* available, so IMHO we
>>> should remove this *iff* we can provide this guarantee.  Otherwise the check is
>>> valid. You see, although the UMH lock was bogus, it did implicitly ask the
>>> question: is it safe for *any* helper to run and make assumptions on the
>>> filesystem then?
>>> 
>>> In lieu of this question being answered the warning is valid given the design
>>> of the firmware API and the having the cache available as a measure to resolve
>>> this race.
>> 
>> I don't understand what you are trying to say here at all.
>> 
>> To be specific, what, if anything, is a problem with the current state
>> of Linus's tree (and the next 4.13-stable release)?
> 
> The warning is issued when drivers issue *new* firmware requests on resume. The
> firmware API cache was designed to enable drivers to easily be able to request
> firmware on resume without concern about races against the filesystem, but in order
> for this to work the drivers must have requested the firmware prior to suspend.
> 
>> If something needs to be fixed, can you make a patch showing that?  Or
>> do we also need to revert anything else as well to get back to a "better
>> working" state?
> 
> I took a look at the driver and it seems that btusb_setup_intel_new() is
> not called after the driver is initialized, rather its called only when
> hci_dev_do_open() is called. Its not clear to me how you can end up calling
> this on resume but not prior to this on a running system. Feedback from
> someone more familiar with bt would be useful.
> 
> I'd have the call for firmware on probe, no processing would be needed, just
> a load to kick the cache into effect would suffice. This may require a bit
> of code shift so its best someone more familiar do this.
> 
> If it confirmed this information is helping avoid these races we can reconsider
> re-instating the warn as a firmware dev debugging aid for developers.
> 
> If the race this warning complained about is indeed possible the same race is
> also possible for other usermode helpers. Its *why* the UMH lock was
> implemented, it however was never generalized.

we can not load firmware on probe() in most cases. The btusb.ko driver establishes the HCI transport. It is setup in probe() and then started in hci_dev_do_open() and if there is a vendor setup routine like btsub_setup_intel_new(), then it is executed. Most Bluetooth controllers (not all, but most) are doing firmware loading over the HCI transport with vendor specific commands.

And yes, if the firmware was already loaded, we would skip requesting it at all. Which means after suspend/resume cycle where the power to the controller is cut, then we are missing the firmware from the cache. Since we never loaded it in the first place.

So yes, we would have to redo parts of the vendor specific handling to always request the firmware, even if we do not need it right now. And frankly that is not obvious to anybody. We seem to have some patches for doing exactly that, but I have not gotten to review them in detail since they deal with vendor specific complex setup handling. Also this affects more than just Intel since all hardware where firmware loading is skipped if there is already firmware loaded are affected.

Regards

Marcel